This invention relates to a methodology for bonding together two microfabrication substrates.
Microelectromechanical systems are devices which are manufactured using lithographic fabrication processes originally developed for producing semiconductor electronic devices. Because the manufacturing processes are lithographic, MEMS devices may be made in very small sizes. MEMS techniques have been used to manufacture a wide variety of transducers and actuators, such as accelerometers and electrostatic cantilevers.
Because MEMS devices are often movable, they may be enclosed in a rigid structure, or device cavity formed between two wafers, so that their small, delicate structures are protected from contamination or atmospheric conditions. Many MEMS devices also require an evacuated environment for proper functioning, so that these device cavities may need to be hermetically sealed after evacuation. Thus, the device cavity may be formed between two wafers which are bonded using a hermetic seal, which encloses the MEMS device in a protective and well controlled environment.
Microfabricated optical devices, such as emitters, reflectors, absorbers, gratings, and the like also exist. They may use or manipulate electromagnetic radiation, Such optical devices may require encapsulation in an optically transmissive device cavity to function effectively. Glass wafers would provide such a cavity, allowing electromagnetic radiation to pass into and out of the device cavity. However, a hermetic seal around a glass cavity typically uses a glass frit adhesive, which may require processing temperatures in excess of 400C to melt and fuse the frit. These temperatures may exceed the temperatures that can be withstood by many of the thin metal layers used to create the optical device. Thus, encapsulation of an optical device in a transparent device cavity which is hermetically sealed has been an elusive goal.
It is possible to bond an optically opaque substrate, such as silicon, to an optically transparent substrate such as glass, with the glass substrate forming a lid over the silicon wafer and microfabricated device. Anodic bonding of a silicon substrate to an optically transparent glass substrate is known, wherein voltage and heat are applied between the glass wafer and the silicon wafer. The voltage applied promotes the growth of the oxide layer between the silicon and the glass, which bonds the substrate materials together.